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Microcline and Hedenbergite in a Syenite from Quebec The photos show hedenbergite (Fe-rich calicclinopyroxene) with microcline. Theclinopyroxenehas higherreliefthan the microcline and a slight greenish color (the difference is visible in PP). It also has poorly developedcleavage. In the XP view, theclinopyroxeneshows 2nd orderinterference colors-- mostly mottled yellow-green; the microcline only first order grays and white. The microcline also displays well developed crosshatchedtwinsin some grains and one set of subparalleltwinsin others. The field of view is about 2.5 mm. | Describe this thin section with the PPL image on the left and XPL image on the right. Identify the minerals present in this. Describe the rock's texture. |
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Olivine, Augite, and Plagioclase in a Porphyry from Near Fish Lake, Oregon This view shows three kinds of phenocrysts in a groundmass of mostlyplagioclase, magnetite (opaque) andclinopyroxene (augite). The phenocrysts are lathes ofplagioclase, several more equant grains of olivine, and two grains ofaugite. In PP light all three are nearly clear, but theaugite(hard to pick out in PP) has a slightly greenish tint. In XP light, the olivine can be seen to have 2nd order pastelinterference colorsthat vary within individual grains. The largest olivine grain, a quarter of the way down from the top center of the photo) shows red, violet, blue and greeninterference colors. The two largestaugitegrains are next to it - one up and to the right and one below and slightly left. Bothaugitegrains show somewhat anomalous yelllow-browninterference colors. The field of view is about 2.5 mm across. | Describe this thin section with the PPL image on the left and XPL image on the right. Identify the minerals present in this. Describe the rock's texture. |
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Quartz and Plagioclase in a Rhyolite Porphyry Here we see large phenocrysts of quartz andplagioclasein a matarix of finer quartz andK-feldspar. The largest grain is quartz and appears to be split along a fracture in the XP view. Many other, smaller, quartz grains are also present. A single large grain ofplagioclasecan be identified by its zebra-stripetwins(XP). Note that in the PP view, the large quartz and feldspar phenocrysts appear to be much less altered than the finer grained groundmass. | Describe this thin section with the PPL image on the left and XPL image on the right. Identify the minerals present in this. Describe the rock's texture. |
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Sanidine and Plagioclase in a Rhyolite Porphyry The photo above shows two large sanidine (K-feldspar) grains, each having a simpletwindown the center. Below the right hand sanidine there is a grain ofplagioclasewith fainttwinsvisible (XP). These phenocrysts are surrounded by a fine grained groundmass of K-feldspar andquartz. This specimen comes from Chaffee County, Colorado. The field of view is about 2 mm. | Describe this thin section with the PPL image on the left and XPL image on the right. Identify the minerals present in this. Describe the rock's texture. |
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Augite and Plagioclase in a Basalt from India This view showshigh relieflight green augite (clinopyroxene), opaque magnetite, and clearplagioclase(PP). In the XP view the augite has up to mid second orderinterference colorsand theplagioclaseshowstwinningin first order black-gray-whiteinterference colors. The augite showscleavage, and the angle betweencleavagesappears to be close to 90oin some grains. The field of view is about 1.5 mm across. | Describe this thin section with the PPL image on the left and XPL image on the right. Identify the minerals present in this. Describe the rock's texture. |
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Hornblende in Syenite from near Cuttingsville, Vermont The photos above show a large grain of hornblende with several other hornblende grains around it.Calciteis present down and to the left of the largest hornblende grain.Plagioclasefills the space between grains. Typical amphibolecleavagecan be seen. The field of view is about 2.5 mm across. | Describe this thin section with the PPL image on the left and XPL image on the right. Identify the minerals present in this. Describe the rock's texture. |
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BiotiteOccurrenceBiotite is widespread and common in a wide variety of igneous and metamorphic rocks and, to a much lesser extent, in some immature sedimentary rocks. CompositionBiotite's chemistry is variable, so optical properties vary as well. The most significant end members are annite (KFe3AlSi3O10(OH)2) and phlogopite (KMg3AlSi3O10(OH)2). IdentificationIn general, keys to identifying biotite are itscolor and pleochrism,cleavage, optical texture and characteristic extinction, and habit. Biotite's features generally make identification unambiguous. However, in some medium-grade metamorphic rocks and volcanic rocks, the features are less prominent. Important properties·Color- generallypleochroic, typically in shades of brown, but also can be yellow, green or red. Common biotite is one of the most strongly colored minerals seen in thin section. For biotite with low Fe-content (phlogopite), coloration is more subtle (clear, light browns and tans).·Habitandcleavage- often appears as tabs or long skinny flakes which may be bent. It shows one excellentcleavage, depending on grain orientation. Rarely, it forms hexagonal crystals.·Birds-eye or pebbly extinctionor wavey pattern ofinterference colors, especially when looking down on flat side of flakes.·Extinction angleis 0 to a few degrees fromcleavage.·Interference colorsmay range up to 2nd order red, but color of mineral may mask them.·Often containspleochroic halosaround small included zircons.·Biaxial (-),2V= 0-25o.·Oftenalterstocholrite. Similar minerals·Light colored phlogopite may be confused withmuscovitebut has a smaller2V.·Brown or greenhornblendeor other amphiboles (images at right) may occasionally be mistaken for biotite, but amphibole has 2 cleavages, often a diamond shaped cross section, and lacks birds-eye/pebbly extinction.PPXP·Stilpnomelane, a brown brittle mica, has higherrelief, lacks birds-eye/pebbly extinction, has poorercleavage, and has a pseudouniaxial interference figure.·Brown or green tourmaline lacks biotite's characteristic extinction,cleavageand habit. It also appears darker/strongercoloredin some orientations.·Chloritehas lower relief and birefringence (often showing anomalous colors) | Describe this thin section with the PPL image on the left and XPL image on the right. Identify the minerals present in this. Describe the rock's texture. |
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Aegirine in an Ijolite from the McClure Mountains, Colorado The photos above show a largetwinnedpyroxene (aegirine) crystal with the classic 8-sided ("stop sign") shape andhigh reliefcompared to the light white-green surrounding material. Thetwindivides the pyroxene into two domains with different optical orientations, easily seen in the XP view. The aegririne is fractured, but with a little imagination it is possible to see twocleavagesthat intersect at near 90o. All the otherhigh-reliefgreen (PP) material is also aegirine. Minor brown (PP)biotiteis present as inclusions. Thelow-relief, nearly clear (PP), material is mostly nepheline but there are also a few small grains ofK-feldspar. The nepheline andK-feldsparare hard to distinguish in the PP view. Nepheline has very lowbirefringence, and so appears almost isotropic in the XP view. The field of view is about 3.5 mm. | Describe this thin section with the PPL image on the left and XPL image on the right. Identify the minerals present in this. Describe the rock's texture. |
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Microcline in a Granite These photos show a large grain of microcline in a granite from near Barre, Vermont. Microcline is characterized by a cross-hatchedtwinning-- here very well developed -- which is visible only in the XP view. Microclinetwinningis sometimes called "tartan plaid" or "Scotch plaid"twinning. Also present in this view arequartz(clear, PP; white to gray, XP) and two flakes of dark brownbiotite. The field of view is 2.5 mm. | Describe this thin section with the PPL image on the left and XPL image on the right. Identify the minerals present in this. Describe the rock's texture. |
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Microcline (left) and Cordierite (right) Twins These two XP views show examples of twinning inmicrocline(left) andcordierite(right).Microclineoften displays two sets of twins, more or less perpendicular to each other.Cordieritetwins in a manner similar toplagioclase, but the twins often pinch out as can be seen here. | Describe this thin section with the PPL image on the left and XPL image on the right. Identify the minerals present in this. Describe the rock's texture. |
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Muscovite in a Biotite Granite The view above shows a large muscovite grain, about 1.5 mm in longest dimension. Also present are clear (PP light), lowbirefringence(XP light)plagioclase(upper left),microcline(lower right) andquartz, and a few dark colored flakes ofbiotite. Thequartz,plagioclase, andmicroclineare best distinguished in the XP view, due totwinningin the feldspars. Note the variableinterference colorsof the muscovite, due to varying thickness of the grain which results in varying amounts of retardation. The rock shown is a granite from near Barre, Vermont. The field of view is about 2 mm. | Describe this thin section with the PPL image on the left and XPL image on the right. Identify the minerals present in this. Describe the rock's texture. |
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Myrmekite and Perthite in a Granite from Colorado The photos above show myrmekite (wormy texture in XP view) and microcline (wavy black-gray-white twinning in XP view). Both are characteristic of K-feldspar and are indistinguishable in PP light. Myrmekite is an intergrowth ofquartzand feldspar. In this sample, the microclinetwinningdoes not produce a cross-hatched appearance, instead haviing a wavey/spindly appearance. Minorbiotite(dark brown to tan) is present along the edges of the photo. | Describe this thin section with the PPL image on the left and XPL image on the right. Identify the minerals present in this. Describe the rock's texture. |
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Plagioclase in a Latite Porphyry from Wolf Creek, Montana The photos above (PP on left, XP on right) show a few large plagioclase grains in a fine grained groundmass that is mostly plagioclase but also contains opaque magnetite (black in PP). Besides beingtwinned, the larger plagioclase shows concentric compositional zonation (seen most easily in PP light). The field of view is about 4.5 mm. | Describe this thin section with the PPL image on the left and XPL image on the right. Identify the minerals present in this. Describe the rock's texture. |
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Biotite in an Andesite The pictures above show a 2 mm wide biotite grain in an andesite from Mineral County, Nevada. PP on the left, XP on the right. The field of view is 4.5 mm. Note the variableinterference colors(XP), caused by variable thickness of the biotite grain. This biotite is armored (surrounded) by magnetite (black,opaquemineral) and lots ofquartzandplagioclase. In this view, we are looking more or less down on a flake, so biotite'scleavageand typical elongate habit do not show well. | Describe this thin section with the PPL image on the left and XPL image on the right. Identify the minerals present in this. Describe the rock's texture. |
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Two XP Views Showing Plagioclase The striped mineral in these two XP views isplagioclase.Plagioclaseis generally a nondescript clear mineral in PP light. When viewed in crossed polars (XP) it often shows twinning as alternating bands of white, black and gray. Rotating the stage causes the different bands to go extinct (turn black) at different times. The different domains in the grains are allplagioclase, but they have slightly different optical orientations, thus giving rise to the striped appearance. Commonplagioclasejust shows twinning in one direction (view on left), but sometimes (view on right) two sets of twins are present. | Describe this thin section with the PPL image on the left and XPL image on the right. Identify the minerals present in this. Describe the rock's texture. |
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Epidote in a Highly Altered Basalt The photos above show epidote that has filled an amygdule in a highly altered basalt. Thecolorof the epidote is typical: a sort of off-color yellow-green. Theinterference colors, too, are classic: various shades of pastels within individual grains. The opaque material around the amygdule is a mixture of glass, hematite andchlorite. Note severalbubblesintroduced when the thin section was made. The field of view is about 3.5 mm. This specimen comes from Keweenaw County, Michigan. | Describe this thin section with the PPL image on the left and XPL image on the right. Identify the minerals present in this. Describe the rock's texture. |
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Chlorite in a Metasedimentary Rock The views above show chlorite (light green in PP; anomalous blue in XP) surrounding a flake ofgraphite(black, opaque). The rock is a metamorphosed shale collected near Poughkeepsie, New York. Most of the other material seen ismuscovite, butgraphiteis scattered throughout in wavy layers, and there is one largequartzcrystal on the top center edge. Field of view is 2 mm across. | Describe this thin section with the PPL image on the left and XPL image on the right. Identify the minerals present in this. Describe the rock's texture. |
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Spinel with Pyroxenes in a Peridotite Xenolith from Kilbourne Hole, New Mexico These photos show ahigh-reliefgreen spinel grain in the upper right (PP). Most of the rest of the view isclinopyroxene(augite). A large grain oforthopyroxene, with slightly greaterreliefthan theclinopyroxene, is on the left edge of the photos. Note that the spinel (isotropic) appears black in the XP view. The field of view is 4.5 mm. | Describe this thin section with the PPL image on the left and XPL image on the right. Identify the minerals present in this. Describe the rock's texture. |
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Orthopyroxene in a Spinel Peridotite Xenolith from Kilbourne Hole, New Mexico These photos show mostly orthopyroxene (light clear to green, PP; grayinterference color, XP). Several small grains ofclinopyroxene(2nd order yellow to blueinterference colors, XP) are also also present -- mostly along the left side of the photos. Note the near 90ocleavage anglein the orthopyroxene. The field of view is 4.5 mm. | Describe this thin section with the PPL image on the left and XPL image on the right. Identify the minerals present in this. Describe the rock's texture. |
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Plagioclase and Augite in a Basalt The photos above show abundant plagioclase lathes (clear - PP; low orderinterference colors- XP) and theclinopyroxeneaugite(almost clear- PP; 2nd order yellow to blue to pinkinterference colors-XP) in a matrix that contains a lot of brown glass. Two vesicles are also present (clear, PP; isotropic, XP) -- one left of center on the bottom edge, and the other near the center of the right-hand side. Some of the plagioclase exhibitstwinning, giving a zebra striped appearance in XP light, although it is hard to see at this scale. This sample is a basalt from the Cima Volcanic Field, southern California. The field of view is about 2.5 mm across. | Describe this thin section with the PPL image on the left and XPL image on the right. Identify the minerals present in this. Describe the rock's texture. |
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Olivine in a Xenolith from the Cima Volcanic Field, California This veiw shows a large grain of olivine surrounded by fine grained magnetite,clinopyroxeneand minorplagioclase. The fine grained material is hard to identify in this view. The field of view is 4.5 mm. | Describe this thin section with the PPL image on the left and XPL image on the right. Identify the minerals present in this. Describe the rock's texture. |
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Clinopyroxene in a Lherzolite from the Cima Volcanic Field, California The photos (3 mm across) show mostly clinopyroxene. The large kite-shaped grain in the center is clinopyroxene. The high reliefshrimp-shaped grain up and to the right of it (showing second order purply-red-orangeinterference colors), and the triangular grain with yellow-greeninterference colorsbelow it, areolivine. Several fractured grains showing first order white to grayinterference colorsareorthopyroxene. One small flake of brownbiotite(PP) can be seen in the lower center. | Describe this thin section with the PPL image on the left and XPL image on the right. Identify the minerals present in this. Describe the rock's texture. |
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Orthopyroxene in an Olivine Websterite Xenolith from the Cima Volanic Field, California These photos show a large grain of orthopyroxene surrounded byclinopyroxene(some of which is near extinction in the XP view). In the XP view, the orthopyroxene shows only first-orderinterference colors-- theclinopyroxeneranges up to upper second order orange to pink (near the top center of the photo). Only one well developedcleavageis visible in this orthpyroxene grain. | Describe this thin section with the PPL image on the left and XPL image on the right. Identify the minerals present in this. Describe the rock's texture. |
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Epidote and Hornblende in a Mafic Schist from near Panoche Pass, California The photos showpleochroichornblende(green hues; PP) and epidote (clear; PP) in a mafic schist. Note the epidote grains show multipleinterference colorscreating zones or concentric rings in most grains (XP). Also note that some of thehornblendeshows a hint of a diamond shape and of amphibole's characteristic 60o-120ocleavage angle. The field of view is about 2.5 mm. | Describe this thin section with the PPL image on the left and XPL image on the right. Identify the minerals present in this. Describe the rock's texture. |
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Chlorite Showing Anomalous Brown Interference Colors The PP view shows light green chlorite, blueglaucophane, clearmuscovite, and scattered smallepidotecrystals in the PP view. In the XP view, the chorite can be identified by its anomalous browninterference colors. Themuscoviteappears as flakes withupper second order interference colors(XP). Theglaucophaneshowsfirst order interference colors(XP). This sample comes from near Panoche Pass, Callifornia. The field of view is about 2.5 mm. | Describe this thin section with the PPL image on the left and XPL image on the right. Identify the minerals present in this. Describe the rock's texture. |
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Chlorite, Glaucophane and Epidote in a Blueschist Greenchlorite, blueglaucophane(an amphibole), clearwhite micaflakes (bottom center and right) and a number of smallhigh-reliefepidote grains (most are wedge-shaped) are visible in PP light. In XP light thechloriteshows anomalousinterference colors, the glaucophane shows 2nd orderinterference colors, in places somewhat masked by the bluecolorof the mineral. The mica flakes show obvious mottled 2nd orderinterference colors.The epidote grains are hard to pick out in XP light This sample comes from near Panoche Pass, Callifornia. The field of view is about 2.5 mm. | Describe this thin section with the PPL image on the left and XPL image on the right. Identify the minerals present in this. Describe the rock's texture. |
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Glaucophane and Epidote with Jadeite (Blueschist) This section contains abundant blueglaucophane, some showing the classic diamond-shaped amphibole cross section andcleavage. Many small grains ofhigh-reliefepidote are visible in PP light but get lost in the XP view. The nearly clear matrix material that encloses theglaucophaneand epidote isjadeite. It displays low orderinteference colors, anomalous in some grains, in the XP view. This sample comes from near Panoche Pass, Callifornia. The field of view is about 2.5 mm. | Describe this thin section with the PPL image on the left and XPL image on the right. Identify the minerals present in this. Describe the rock's texture. |
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Chlorite Replacing Biotite The photos to the left show chlorite as an alteration patch in a grain ofbiotite(PP on far left, XP on immediate left). Some additional chlorite (light green in PP) is just to the right of the bottom right corner of thebiotite. Note the light greencolorof the chlorite and the anomalous (gray to dark inky blue)inteference colors. The surrounding minerals are mostlyquartzand feldspar. The field of view is about 2.5 mm. | Describe this thin section with the PPL image on the left and XPL image on the right. Identify the minerals present in this. Describe the rock's texture. |
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Quartz, Plagioclase and Orthoclase in a Quartz Monzonite from near Garfield, Colorado It can be difficult to distinguish quartz and the different feldspar varieties from each other. Here, the quartz can be seen to be fresher (unaltered) in the PP view; this is often a key distinction. One large quartz grain is near the center of the view, and two others are near the left hand side. The largeplagioclasegrain in the upper right istwinnedin an unusual way, but the smallplagioclasedown and to the left of the quartz shows more classicpolysynthetic twinning(appearing as vague black and gray stripes in XP). Most of the bottom part of the thin section isorthoclase-- here it is near extinction (XP). A single,high relief,sphenecan be seen near the lower left corner (PP), and several small grains of magnetite (opaque; black in both views) are present. The field of view is about 2.5 mm. | Describe this thin section with the PPL image on the left and XPL image on the right. Identify the minerals present in this. Describe the rock's texture. |
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Hornblende Replaced by Chlorite, Epidote and Magnetite in a Granite The diamond-shaped original crystal washornblendebut it has been replaced by other minerals. In PP light,chloriteand epidote both appear light green (but thechloritehas a more "micaceous" character). Under crossed polars (XP) they are distinguished because the chlorite shows anomalous green-grayinterference colorswhile the epidote shows upper second orderinterference colors. Th opaque mineral is magnetite;quartzandfeldsparsurround the amphibole grain. This sample comes from near Garfield, Colorad. The field of view is about 2 mm. | Describe this thin section with the PPL image on the left and XPL image on the right. Identify the minerals present in this. Describe the rock's texture. |
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Epidote, Dolomite and minor Pholgopite in a Marble This is a marble from Essex County, New Jersey. Thetwinneddolomite is clear (PP) and shows high order, almost white,interference colors(XP). Dolomite, like calcite, is one of the few minerals that showstwinsin both PP and XP.Epidote, also present, has greaterreliefthan the surrounding dolomite and has a very light pistachio greencolor(PP). The epidote shows a typical (for epidote)interference colorpattern - blotchy high order colors of many hues. Several flakes of brown-greenphlogopite(PP) can also be seen. The field of view is about 2.5 mm across. | Describe this thin section with the PPL image on the left and XPL image on the right. Identify the minerals present in this. Describe the rock's texture. |
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Serpentine and Talc in a Serpentenite (Verde Antique) This photomicrograph, about 2 mm across, is of a serpentenite from Windsor County, Vermont. Most of the photo shows antigorite, perhaps the most common variety of serpentine (clear to light greencolorPP,1st order interference colorsXP). The material forming a sickle-shaped patch near the center of the view, showinghigher reliefand veryhigh order interference colors, istalc. Some patchy magnetite (opaque) can be seen near the right edge. | Describe this thin section with the PPL image on the left and XPL image on the right. Identify the minerals present in this. Describe the rock's texture. |
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Wollastonite, Diopside and Garnet in a Skarn This photograph shows a large grain of wollastonite (clear, diamond shaped, center of photo), a smaller grain ofdiopside(higher relief, upper left part of photo) andgarnet(isotropic; brownish PP; extinct XP). This rock comes from a classic wollastonite deposit near Willsboro, New York. The field of view is about 2 mm. | Describe this thin section with the PPL image on the left and XPL image on the right. Identify the minerals present in this. Describe the rock's texture. |
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Muscovite in a Mica-Kyanite Schist The PP view above shows clear muscovite, light brownbiotite, and a few grains of clear,high relief,kyanitenear the top center. Although the flakey habit, typical of micas, can be seen here, it is not as pronounced as in the previous photos because this view looks down on the flat surface of muscovite flakes instead of at flake edges. The rock shown is a mica schist from Manhattan, New York. Field of view is 2 mm. | Describe this thin section with the PPL image on the left and XPL image on the right. Identify the minerals present in this. Describe the rock's texture. |
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Plagioclase in Granodiorite from St. Cloud, Minnesota The photos above show several large grains of plagioclase, a grain of faintly twinnedK-feldspar, several grains ofquartz, and several brown-green grains ofbiotitein a granodiorite from the Minnesota River Valley. In the PP view, the plagioclase, although clear, shows signs of incipient alteration (giving it a sort of rough scratchy appearance). In the XP view, the plagioclase shows zebra stripetwinning. TheK-feldsparappears similar to the plagioclase but itstwiinningis less well developed. Thequartz, similar to both feldspars, shows undulatory extinction and notwinning. The field of view is about 4.5 mm across. | Describe this thin section with the PPL image on the left and XPL image on the right. Identify the minerals present in this. Describe the rock's texture. |
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Talc with Minor Tremolite in a Schist This is a talc-tremoliteschist from the Adirondack Mountains, New York. Most of this view is talc; only minortremoliteis present in the lower right of the photo (showing 2nd order yellowinterference colorsin XP). Thetremolite, difficult to see in the PP view, stands out in the XP view because theinterference colorsare significantly lower order than talc's. The field of view of these photos is about 2.5 mm. | Describe this thin section with the PPL image on the left and XPL image on the right. Identify the minerals present in this. Describe the rock's texture. |
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Talc with Tremolite in a Schist This is a talc-tremoliteschist from the same location as the previous photos. Thetremoliteis clear in PP light and has 2nd orderinterference colorsin the PP view. The talc, which has a dingy greencolorin the PP view shows extremely high orderinterference colors-- almost "pearl white" in the XP view. The field of view of these photos is about 2.5 mm. This thin section was poorly made. Many smallbubbleswere trapped in the epoxy when it was prepared. They appear as small round circles in these pictures. | Describe this thin section with the PPL image on the left and XPL image on the right. Identify the minerals present in this. Describe the rock's texture. |
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Garnet, Omphacite and Titanite in an Eclogite These photos show a large euhedralgarnet(isotropic, in the lower left of view) in a sea of mostlyomphacite(a high pressuresodic pyroxene), titanite, and a few flakes ofwhite mica. Thegarnetis slightly altered tochloritealong its edges; note theanomalous interference colorsin the XP view. Theomphaciteis pale green (PP) withinterference colorsthat range up to first order red (XP). In a few places, wherehigher-order colorscan be seen, theomphaciteis being replaced byhornblende. The titanite crystals are small, have veryhigh relief(PP), and appear as irregular elongate grains, some of which have acute terminations. Several flakes ofwhite micaare present -- they are the clearest grains visible in the PP view. This specimen is from Sonoma County, California. The field of view is about 1.5 mm across. | Describe this thin section with the PPL image on the left and XPL image on the right. Identify the minerals present in this. Describe the rock's texture. |
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Twinned Andalusite These views show atwinnedandalusite crystal, partially altered tosericite. Andalusite that shows a cross in end view is termedchiastolite. The cross appears, in part, because of dark included organic matter. The field of view is about 3.5 mm. | Describe this thin section with the PPL image on the left and XPL image on the right. Identify the minerals present in this. Describe the rock's texture. |
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Twinned Andalusite (left), Diopside (right) and Calcite (right) Both of these are XP views. The left hand view showsandalusitethat has been twinned to give a cross (calledchaistolite). Twins of this sort, where different crystals seem to grow into each other, termedpentration twins, are rarely seen in thin section. More commonly twins are seen within a single grain. The view on the right has a largediopsidegrain in the center. Faint stripes can be seen running across the grain. These are twins -- and the stripes show because the different domains in the grain have slightly different optical orientations. The mineral around thediopsideiscalcite. It, too, is twinned. The twins show as pastel stripes in some places. | Describe this thin section with the PPL image on the left and XPL image on the right. Identify the minerals present in this. Describe the rock's texture. |
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Staurolite in a Quartzite This view shows a large mass of yellow starurolite surrounded by clearquartz(PP). The staurolite has a radiating habit, not uncommon for this mineral. The minoropaquemineral is magnetite, and hematite stains some of the cracks. This sample comes from Rio Arriba County, Mexico. The field of view is about 2.5 mm across. | Describe this thin section with the PPL image on the left and XPL image on the right. Identify the minerals present in this. Describe the rock's texture. |
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Kyanite and Quartz, with Minor Muscovite This veiw includes two kyanite grains/aggregates, one a large blade showingcleavageand the other a patchy mass showing nocleavage. The kyanite is surrounded byquartz. Thequartzis clear with lowreliefand low orderinterference colors, while the clear kyanite has highreliefandinterference colorsrange up to first order yellow. (In some thin sections, kyanite'sinterference colormay be first order reds.) Note the undulatory extinction displayed byquartz. Several flakes ofmuscovite(clear withlow reliefand uper second orderinterference colors) can be seen just to the right of the kyanite. This view is about 1 mm across. | Describe this thin section with the PPL image on the left and XPL image on the right. Identify the minerals present in this. Describe the rock's texture. |
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Quartz with Rutile and Muscovite Most of this view shows quartz. It is characterized bylow relief, undulatory extinction and low orderinterference colors. The brownish,high-reliefgrains (PP) are rutile. Rutile has very highbirefringencebut the color of the grain is so strong that itsinterference colorsdon't show (XP). A number of small flakes ofmuscoviteare also present. They cannot be distinguished from thequartzin the PP view, but in XP they are marked by their flakey shape and high second orderinterference colors. This sample comes from New Ogilby, California. The field of view is about 1.5 mm. | Describe this thin section with the PPL image on the left and XPL image on the right. Identify the minerals present in this. Describe the rock's texture. |
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Biotite With Anthophyllite This sample is a biotite-anthophyllite-cordieriterock from near Guffey, Colorado. In this view, one large flake of biotite dominates the top half of the photos. It is light brown (PP). In XP light it shows a pebbly texture and mostly second order greeninterference color(XP). Most of the clear mineral matter beneath the biotite is anthophyllite, an amphibole. Minorcordieritecannot be easily picked out at this magnification. The anthophyllite showslow second order interference colors(XP) and has a tabular to bladed appearance. The field of view is about 2 mm. | Describe this thin section with the PPL image on the left and XPL image on the right. Identify the minerals present in this. Describe the rock's texture. |
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Glaucophane (and epidote) in a Blueschist Most of this thin section consists of blades of the blue amphibole glaucophane. In XP light, itsinterference colorsare somewhat anomalous because thecolor of the mineralhas added to the normal upper first-orderinterference colors. The light green mineral with relativelyhigh reliefisepidote(PP). Note that itsinterference colorsare high-order in some grains and appear anomalous in others. This specimen comes from Sonoma County, California. The field of view is 3.5 mm. | Describe this thin section with the PPL image on the left and XPL image on the right. Identify the minerals present in this. Describe the rock's texture. |
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Actinolite Schist from Near Chester, Vermont The photos above show mostly thepleochroicgreen amphibole actinolite and clearquartz(PP). Also present is blackopaquemagnetite. In the XP view, the deep color of the actinolite partially masks theinterference colors, making them somewhat anomalous. A few of the amphibole grains show the classic 60o-120ocleavage angles. The field of view is about 2.5 mm across. | Describe this thin section with the PPL image on the left and XPL image on the right. Identify the minerals present in this. Describe the rock's texture. |
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Calcite (or Dolomite) in a Marble The views above show calcite (or dolomite) in a marble from the Adirondack Mountains, New York. The field of view is about 2 mm across. Minorgraphite(opaque) is also present as long skinny flakes in the carbonate. Most of this view is one large grain showing two sets oftwins. Calcite is invariably twinned but thetwinsdo not show at all orientations. The largest grain is oriented so many of thetwinlamellae are at extinction (black in XP). Very high orderinterference colors-- high order pastels -- show along the (not extinct)twinlamellae. The large grain to the upper left is oriented such thattwinsdo not show. This grain hasinterference colorsthat are of such high order that they appear white. | Describe this thin section with the PPL image on the left and XPL image on the right. Identify the minerals present in this. Describe the rock's texture. |
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Tremolite in a Marble This sample is a marble from the Adirondack Mountains, New York. Almost all of the field of view (about 2 mm across) is tremolite. The large grain in the center (viewed nearly down the c-axis) shows the classic amphibole cross section andcleavage. Smaller grains around it have their long axes in the plane of the thin section and so appear as long bladey crystals. | Describe this thin section with the PPL image on the left and XPL image on the right. Identify the minerals present in this. Describe the rock's texture. |
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Diopside in a Dolmitic Marble These photos show diopside in a marble from the Adirondack Mountains, New York. Field of view is about 2 mm. The football shaped diopside grain in the center shows classic near 90oangle betweencleavages- diagnostic of pyroxene. It also shows incipienttwinning(XP). Diopside'sinterference colorsrange up to mid second order but in views that show twocleavagestend to be lower. This diopside is in a matrix ofdolomite. Here the dolomite appears slightly pinkish because the thin section was etched and stained with alizarin red stain to help distinguish calcite from dolomite (calcite stains a darker red color). The dolomite istwinnedand shows very high orderinterference colors; they appear as pastels, in places almost pearly white. | Describe this thin section with the PPL image on the left and XPL image on the right. Identify the minerals present in this. Describe the rock's texture. |
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Biotite With a Halo Around a Zircon Inclusion This sample is a biotite-cordierite-orthopyroxenegneiss from near Sioux Lookout, Ontario. The field of view, about 2 mm across, is dominated by olive-green-brownbiotite. The largest grain ofbiotitecontains a pelochroichalo(burn mark) around an included zircon. Although hard to identify in this view, the clear,high reliefgrains to the left and beneath the biotite areorthopyroxene. The large clear grain to the lower right iscordierite. which, in this view, looks likequartz. Thebiotitegrain in the lower left shows one goodcleavage, but the large grain in the center does not because the view is more or less perpendicular to thecleavage. It does, however, show a partially developed "birds eye" pattern in XP light. | Describe this thin section with the PPL image on the left and XPL image on the right. Identify the minerals present in this. Describe the rock's texture. |
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Cordierite, Orthopyroxene and Biotite from near Kazabazua, Quebec The four photos above are of a spectacular rock from near Kazabazua, Quebec. It contains primarily cordierite andorthopyroxenewith lesser amounts ofbiotite. The cordierite istwinned(and in XP looks a lot likeplagioclase) and contains pleochorichalosaround zircon inclusions. The halos appear as "burn" marks in PP light. Theorthopyroxeneis blocky and fractured, showshigh relief(PP), and has first orderinterference colors(XP). It ispleochroic, with color ranging from light pink to light green.Biotiteis alsopleochroicand here exists as flakes in various shades of brown (PP) and has upper 2nd orderinterference colors(XP). | Describe this thin section with the PPL image on the left and XPL image on the right. Identify the minerals present in this. Describe the rock's texture. |
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Cordierite, Biotite and Quartz from near Ft. Coulonge, Quebec Imperfectlytwinnedcordierite makes up most of this view (large grain at center of photo). A number of flakes ofbiotiteand a few grains ofquartzsurround the cordierite. The smallhigh-reliefmaterial in the center of the cordierite grain are needles ofsillimanite. The field of view is about 2.5 mm across. | Describe this thin section with the PPL image on the left and XPL image on the right. Identify the minerals present in this. Describe the rock's texture. |
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Cordierite, Garnet and Biotite This sample contains largetwinnedcordierite (left side) and a highly fracturegarnetwith somequartzinclusions (right side). Minorbiotiteandquartzare also present. The cordieriteinterference colorsandtwins, visible in the XP view, make this cordierite appear similar to someplagioclase. Thegarnetis isotropic and so is black in the XP view. These photos are from the same thin section as the preceding ones. | Describe this thin section with the PPL image on the left and XPL image on the right. Identify the minerals present in this. Describe the rock's texture. |
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Garnet and Chloritoid in a Muscovite Schist from near Poughkeepsie, New York These views shows a blade of choritoid (right center). Note its relativelyhigh relief,cleavage(seen in PP),twinning, andfirst order interference colors(in XP). Also present are fine-grainedmuscoviteintermixed with some minorgraphite, and a large euhedralgarnet(isotropic in the XP view). A small flake ofbiotiteis between thegarnetand the chloritoid. The fracturedgarnetcontains two flakes ofgraphite, visible in the PP view. The field of view is 3.5 mm across. | Describe this thin section with the PPL image on the left and XPL image on the right. Identify the minerals present in this. Describe the rock's texture. |
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Garnet, Paragonite and Chlorite The photos show a large garnet (high relief, isotropic) near the center of the photograph. Most of the material to the left of the garnet isparagonite(high orderinterference colors, XP), a white mica often indistinguishable frommuscovitewithout chemical analysis. Some flakes ofchloritecan be seen within and to the right of the garnet. The chlorite has lowerreliefthan the garnet and shows grayinterference colors(XP).Quartz,graphite, and minorbiotiteare also present. This sample comes from woutheastern Vermont. The field of view is about 3 mm across. | Describe this thin section with the PPL image on the left and XPL image on the right. Identify the minerals present in this. Describe the rock's texture. |
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Kyanite in Schist from Near Flin Flon, Manitoba This view has one large blade of kyanite at the center. It is surrounded mostly by (twinned)plagioclase, minorquartz, and flakes ofpleochroicbrownbiotite. A single largegarnet(isotropic) is on the right hand edge of the view, just below center. Theplagioclaseandquartzare difficult to tell appart in at this magnification. The field of view is about 2.5 mm across. | Describe this thin section with the PPL image on the left and XPL image on the right. Identify the minerals present in this. Describe the rock's texture. |
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K-feldspar, Quartz and Biotite in a Granite This view shows two large flakes of brownish greenbiotiteand minorhornblende(darker smaller grains) near the left edge; thehornblendedoes not have a distinctive appearance. Thebiotitehas a pebbly texture and micaceouscleavage. The right half of the view contains a large grain of clear to cloudy K-feldspar. The clear grains withlow birefringenceleft of the K-feldspar arequartz, and minorplagioclase(feldspar) is in the extreme lower left. The feldspars andquartzare difficult to distinguish in the PP view, although the feldspars are a bit cloudier due to incipient alteration. In the XP view, poorly developed microcline twinning gives the K-feldspar an irregular mottled appearance, clearly distinguishing it from thequartz. This sample is a granite from the Minnesota River Valley, Minnesota. The field of view is about 2 mm across. | Describe this thin section with the PPL image on the left and XPL image on the right. Identify the minerals present in this. Describe the rock's texture. |
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Plagioclase, Hornblende, Quartz and Biotite in a Gneiss from near Flin Flon, Manitoba The clear minerals (PP) in this thin section are plagioclase (twinned) andquartz(not twinned). The green mineral (PP) ishornblendeand the brown mineral isbiotite(PP). The biotite ispleochroic, and some grains have a color similar to that of thehornblende. The two can be tough to distinguish, but biotite has a more flakey (micaceous) habit. Note the complextwinningin the plagioclase; there are two sets oftwinsnearly perpendicular in the largest grain. In other grains thetwinningis discontinuous. The field of view is about 2.5 mm. | Describe this thin section with the PPL image on the left and XPL image on the right. Identify the minerals present in this. Describe the rock's texture. |
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Staurolite with Fibrous Sillimanite (Fibrolite) This sample, from near Poughkeepsie, New York, contains a large yellow mass of staurolite on the left hand side (PP). The material to the right of the staurolite is mostly a mix of granularquartzand fine fibroussillimanite. Some of thesillimaniteshows upper second orderinterference colorsbut most is so fine grained thatinterference colorsare hard to see. Minormuscoviteis also present. The field of view is about 3.5 mm. | Describe this thin section with the PPL image on the left and XPL image on the right. Identify the minerals present in this. Describe the rock's texture. |
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Staurolite, Muscovite and Graphite This sample, from western Vermont, contains a large somewhat moth-eaten euhedral yellow staurolite surrounded by alower-reliefmat of clearmuscoviteand scatteredopaquegraphite(PP). In PP light, the staurolite has low orderinterfernce colors(the view is nearly down an optic axis) while themuscoviteshows upper order pastels of various hues. The field of view is about 3.5 mm. | Describe this thin section with the PPL image on the left and XPL image on the right. Identify the minerals present in this. Describe the rock's texture. |
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Staurolite This sample contains staurolite mounted in epoxy. The largest staurolite (2 mm across) contains manyquartzinclusions, showing a classic "Swiss cheese" texture, typical of staurolite. | Describe this thin section with the PPL image on the left and XPL image on the right. Identify the minerals present in this. Describe the rock's texture. |
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Biotite-Sillimanite Schist with Graphite, from the Beartooth Mountains, Montana This photomicrograph, about 2 mm across, is of graphite in abiotite-sillimaniteschist. The graphite isopaque(black in both PP and XP), flakey, and is intimately associated with the biotite. Thebiotiteis brown (PP) and showssecond order interference colorsin the XP view. Thesillimaniteis relatively fine grained, showinghigh relief. The largest "patch" ofsillimaniteis in the upper left of the photo. Other, coarsersillimanitegrains are along the bottom edge, and show up tosecond order blue interference colorsin the XP view. The clear material that is notsillimanite, isquartzandK-feldspar. However, it is difficult to distinguish the two minerals in these views, and some of thequartzappears isotropic due to grain orientation (although it is not). The field of view is about 2 mm across. | Describe this thin section with the PPL image on the left and XPL image on the right. Identify the minerals present in this. Describe the rock's texture. |
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Biotite-Sillimanite Schist , from the Beartooth Mountains, Montana The PP view showsbiotitein various shades of brown, clearquartz, and light yellow browntwinnedrutile.The rutile is rimmed by an opaque mineral, probably magnetite. Minorsillimaniteis present as onehigh-reliefmass near the right side, just below center. One grain of clearplagioclaseis just above the largest rutile. In the XP view, the large black grain isquartzthat happens to be near extinction.Biotiteshows typical second orderinterference colors. The rutile shows no distinctinterference colorsbecause the color of the grain masks theinterference colors. | Describe this thin section with the PPL image on the left and XPL image on the right. Identify the minerals present in this. Describe the rock's texture. |
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Biotite-Sillimanite Schist with Graphite, from the Beartooth Mountains, Montana This photomicrograph, about 1 mm across, is of the samebiotite-sillimaniteschist shown above. The PP view shows abundant fine grained, clear,sillimaniteneedles and blades. Brownbiotite, some associated withgraphite, and clearquartzare also present. In the XP view, thebiotiteshows typicalsecond order interference colorsand thequartzshows first orderinterference colors. The fine needlysillimanitealso showsfirst order colors, but a few largersillimanitegrains (oriented so the long dimension is not parallel to the slide) showsecond order colors. | Describe this thin section with the PPL image on the left and XPL image on the right. Identify the minerals present in this. Describe the rock's texture. |
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Grraphite in a Biotite Schist from Western Massachusetts Here we see mostlybiotiteflakes. They are spotted with small grains of graphite, which isopaqueand shows a somewhat "flakey" habit. Thebiotiteispleochroicfrom nearly clear to medium brown (PP) and shows mica's characteristiccleavage. It also displays manyhalosaround radiation burns, probably centered on included zircon grains. The clear mineral (PP) isquartz. The field of view is about 2.5 mm. | Describe this thin section with the PPL image on the left and XPL image on the right. Identify the minerals present in this. Describe the rock's texture. |
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Garnet Schist from Western Massachusetts Most of this view is garnet (clear, PP; istropic, XP). it contains inclusions ofquartzand ilmenite (opaque) and is surrounded byquartzand some grains of brownbiotite. Additionally, there are some very small grains ofsillimanitebelow and above the garnet (high relief, PP) but they are difficult to pick out at this scale. The field of view is about 2.5 mm. | Describe this thin section with the PPL image on the left and XPL image on the right. Identify the minerals present in this. Describe the rock's texture. |
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Quartz Adjacent to Biotite and Sillimanite These photos are dominated by a large grain ofquartz. It is clear (PP), slightly fracture, and displays first order grayinterference colors. Other, smaller,quartzcrystals are scattered around the periphyry. A few small brown (PP)biotiteflakes are also present but are lost in the XP view. At the bottom, mostly on the left side, the blockyhigh-reliefcrystals are sillimanite. Although sillimanite typically forms needles, here we are looking down the axis of the needles so we see a square cross-section. Note that there is one diagonalcleavagecutting across the square sillimanite end sections. This sample comes from Western Massachusetts; the field of view is about 1.5 mm. | Describe this thin section with the PPL image on the left and XPL image on the right. Identify the minerals present in this. Describe the rock's texture. |
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Biotite This view showsbiotite, pleochroic from tan to brown. The two views show the effect of a 90ostage rotation. This is typical color and pleochroism for biotite, but some biotites are pleochroic in other colors, including green and red. | Describe this thin section with the PPL image on the left and XPL image on the right. Identify the minerals present in this. Describe the rock's texture. |
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Rutile in a Whiteschist from the Dora Maira Massif, near Parigi, Italy The views above show several large grains of brownish rutile (PP). The rutile is surrounded by mostlyquartz(on the left and bottom of the photo) and several flakes ofclear micaon the right. Rutile has very highbirefringence. Just a hint ofhigh order pastel interference colorscan be seen here (XP). Although not distinctive in these photos,kyaniteis present just above the two large rutile grains, and also to the left of the smaller grain near the center of the field of view. The kyanite has slightly higherreliefthan surrounding minerals, but otherwise is hard to pick out. It is clear (PP) and shows first order grayinterference colors(XP). The field of view is 1.5 mm across. | Describe this thin section with the PPL image on the left and XPL image on the right. Identify the minerals present in this. Describe the rock's texture. |
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Hornblende Thishornblendeshows pleochroism from green to blue-green. Hornblende is generally pleochroic, often in shades of green, blue, brown, and less commonly red or yellow. This example is in shades of green only, but brown hues are generally more common. | Describe this thin section with the PPL image on the left and XPL image on the right. Identify the minerals present in this. Describe the rock's texture. |
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Exsolution in Feldspar and Pyroxene The views above, both XP, show two examples of exsolution. The one on the left shows a largeK-feldspargrain with a single twin down its center. Included in theK-feldsparare stringers ofplagioclase.K-feldsparwith unmixedplagioclaseis calledperthite. TheK-feldsparandplagioclaseexisted as a single solid solution mineral at high temperature but unmixed due to cooling. The view on the right is of aclinopyroxenegrain. At high temperatures, pyroxenes with intermediate Ca/Mg values are stable. Upon cooling they may unmix into two pyroxenes -- one Ca-rich and the other Ca-poor -- yielding a striped grain like the one shown. The view of perthite (left) comes from the Unversity in Lille, France: http://www.univ-lille1.fr/geosciences/cours/cours_mineralo/cours_mineralo_3.html. The view on the right comes from the University of Melbourne: http://jaeger.earthsci.unimelb.edu.au/Images/Mineralogical/Textures/main.html. | Describe this thin section with the PPL image on the left and XPL image on the right. Identify the minerals present in this. Describe the rock's texture. |
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Sapphirine Sapphirine is a remarkable blue mineral that often shows extreme pleochroism. These views are of the same part of a thin section, but the stage was rotated, so they are 90odifferent in orientation. The sapphirine goes from deep blue on the left to a lighter blue on the right. Orthopyroxeneis also present. It has a light pinkish or orange color on the left and is greenish on the right. | Describe this thin section with the PPL image on the left and XPL image on the right. Identify the minerals present in this. Describe the rock's texture. |
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